In a typical electric power steering (EPS) system, a hand wheel is connected to a shaft, which comprises an upper shaft and a lower shaft connected by a torsion bar. The upper shaft connects to the hand wheel and the lower shaft connects to an intermediate shaft that ultimately connects to the rack and pinion gear of a vehicle. When the hand wheel is turned, the upper shaft rotates and a torque sensor measures the angular displacement of the torsion bar. The torque sensor is typically located at the interface between the upper and the lower shaft, which is also the location of the torsion bar. The type of torque sensor typically used has been a contacting type, which requires use of a torsion bar to measure the amount of twist on the torsion bar. The torque sensor sends a signal to the controller, which then sends a signal to the motor to begin operating. The motor powers a gear mechanism, which provides assistance in turning the lower shaft and ultimately the road wheels.
A drawback of such torque sensors that rely on the relative rotational displacement of an upper and lower shaft is that they generate hysteresis, which is a lagging effect, and torque ripple, both effects being detrimental to the feel of the power assist steering system. Hysteresis is generated, e.g., from the sensor, the torsion bar itself, bearings on the upper and lower shafts, and any misalignment of the shafts. The amount of hysteresis of the sensor, torsion bar, and bearings can be 0.5 Nm or larger. Hysteresis in these elements generate a torque ripple effect which can be felt at the handwheel as an uneven resistance or periodic pulling effect.
Disclosed is a method for controlling an electric power assist steering system with low hysteresis and torque ripple by sensing torque in a steering shaft at a point along said steering shaft between a hand wheel and a mechanical connection to an electric motor, wherein the sensing includes sensing a magnetic field direction and intensity.